CHICAGO (AP)  It isn't SARS, but infectious disease specialists are trying to learn more about a recently discovered virus that some think may be the culprit in many unexplained respiratory illnesses around the world.

The exact prevalence of human metapneumovirus isn't known, but Yale University researchers recently found it in 6.4% of retested lab samples from 296 children with respiratory symptoms in late 2001 and early 2002, according to a study published Monday in the June edition of Pediatrics.

It was also discovered after the fact in about 4% of retested specimens taken from Rochester, N.Y.-area adults in 1999 through 2001, University of Rochester researchers reported in a Journal of Infectious Diseases article earlier this year.

While those sites are the only published evidence of the virus in the United States, it is thought to be far more prevalent and also has been found in Canada, Europe and Australia.

Like severe acute respiratory syndrome, human metapneumovirus has been associated with flu-like infections and pneumonia-like symptoms, but it does not appear to be as infectious as SARS, said Jeffrey Kahn of Yale University medical school, senior author of the Pediatrics study.

None of the patients in the Yale or Rochester reports died.

Symptoms may include nasal congestion, wheezing, and lung inflammation, and the symptoms may range from mild to serious enough to require hospitalization, Kahn said.

"It can be added to the list of newly emerging human respiratory viruses and really represents another challenge for the medical community," Kahn said.

It belongs to the paramyxovirus family, which also includes a common bug called respiratory syncytial virus, or RSV. SARS is believed to be caused by a different pathogen.

The average child gets about 10 respiratory infections by age 1 and many more throughout childhood. Many are caused by cold viruses, influenza or RSV, but doctors aren't able to pinpoint a cause in about a third of the cases, Kahn said.

Human metapneumovirus may be the culprit in many such cases, he said.

The virus, nicknamed hMPV, was first identified about two years ago in the Netherlands but likely has been around for a long time, said researcherAnn Falsey of the University of Rochester.

It has less conspicuous features under the microscope than other viruses and may simply have gone undetected or misdiagnosed before the Dutch discovery, Falsey said.

"This may be one more piece to the puzzle as to what all these respiratory infections are," she said.

The Yale and Rochester researchers detected the virus in specimens retested after the Dutch discovery.

Early efforts to find a potential vaccine for the virus are under way, said Larry Anderson, a respiratory disease specialist at the federal Centers for Disease Control and Prevention.

There are no commercially available tests for hMPV; diagnoses can only be made at special research labs, Kahn said.

More research is needed to learn how common it is, whether it occurs seasonally like influenza, and to determine the range of symptoms it can cause and what treatments might help, Kahn said.

"There's a lot more that we don't know than we do know right now," he said.

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